People who like to get things done do not like if they cannot make decisions. You are with a client and he wants something. You know it is doable, and you want to say yes. So you do.
Later on, you come back to your colleagues with the agreement, and they will tell you, that what you have promised will not be done. Either it does not make sense, or it is too expensive, or there is a better solution.
Now you are in a defensive mode. As you do not want to go back to the client and renegotiate to not look dumb, you will try to push it internally. Even if you succeed, it is not a win. If you do not win, you have to get back to the client anyway.
This all could have been avoided by using standard hostage negotiator tactic – “This is above my pay grade. I will need to check with my boss. While you wait, can we send you some pizza in so you are not hungry?”.
You have just gained some time to think.
Even if you were able to make the call, you do not want to make it under pressure. Anything you are even in the slightest not sure about should be delayed and discussed with someone before you commit to it.
Even if you are the CEO, you should say, that you need to check with your CTO. Or Product Manager. Or Head of Sales. Rarely will the partner refuse to give you a few days or at least few hours.
And if they do, you should think twice before committing, because they want you to make decisions under pressure. And usually that means they want to screw you over.